The operation of an electrohydraulic servovalve (hereinafter servovalve) of the type for which the present invention is particularly important is set forth in detail in U.S. patent application Ser. No. 328,058 entitled Electronic Compensator For An Electrohydraulic Servovalve, by Applegate et al., filed on Dec. 7, 1981, and assigned to the same assignee as the present invention. This Applegate disclosure is incorporated by reference herein for a thorough exposition of the operation of a servovalve.
In brief, the operation of a servovalve includes the employment of a drive system which functions to supply a constant drive signal to the armature of a torque motor of the servovalve. In response to the drive signal, the motor develops a force or torque which is transmitted from the armature to a drive arm. Movement of the drive arm results in an input motion to an hydraulic spool valve. The input motion to the hydraulic spool valve, in turn, regulates a flow of hydraulic fluid, for example oil, to either side of a main cylinder. The resulting difference in pressure on a piston enclosed in the cylinder causes motion of an output shaft.
The energy developed by the motion of the output shaft, for relatively large servovalves (e.g. greater that 3 GPM rated flow), is approximately 5 to 10 joules. This is a significant amount of energy. It is advantageous, therefore, that servovalves be provided with safety features so that in the event of an electrical, hydraulic or mechanical error in the equipment that externally services the servovalve, the servovalve can be quickly and safely de-energized. In this way, it is possible to provide a safe environment for the operator of a servovalve and prevent damage to any auxiliary equipment connected to the output shaft.
One attempt to provide a servovalve with safety features includes the use of an abort valve. The abort valve becomes active when an external error is detected, and functions to remove the hydraulic fluid supply pressure. This action, in turn, eliminates the pressure on the piston so that the motion of the output shaft ceases.
Abort valves have been satisfactorily employed with servovalves which have narrow bandwidths, typically less that 10 hz. For these servovalves, the rise time, as conventionally defined, is approximately 35 milliseconds. Consequently, in order to be effective, the abort valve needs to have a reaction time of less than 35 milliseconds. Since commercially available abort valves have reaction times on the order of 20 milliseconds, they are suitably employed with narrow bandwidth servovalves.
A state-of-the-art servovalve, on the other hand, extends the bandwidth to at least 100 hz. Accordingly, the reaction time of the servovalve is reduced to less that 3.5 milliseconds. It follows, therefore, that the reaction time of the abort valve should also be on the order of 3.5 milliseconds.
As indicated above, however, abort valves which have reaction times appreciably greater than 20 milliseconds are not commercially available. Hence, there is a need for a safety apparatus that can be employed with a state-of-the-art servovalve. The present invention fulfills this need by providing a safety apparatus associated with the drive system of a servovalve. The safety apparatus of the present invention is successfully employed with an extended bandwidth servovalve, since the reaction time of this safety apparatus is less than 50 nanoseconds.
The present invention is particularly suitable for employment with a relatively large, two-stage servovalve. The safety apparatus ensures that, upon the occurrence of an external error, the servovalve is quickly and safely de-energized. Moreover, upon the correction of the external error, the present invention safely restores the normal operation of the servovalve.